Granite Paving Slip Resistance: A Grade You Measure, a Finish You Buy
Granite paving slip resistance is measured, not manufactured in a shed. The grade on the drawing and the finish on the quotation are two different languages, and nobody translates between them. Three test methods return three numbers that do not convert, and a dry reading says nothing about the surface after rain. What the factory can actually guarantee is the surface process and a sealed reference sample.
Granite paving slip resistance: why do the grade and the finish never line up?
Almost every failed enquiry breaks at the same point. The drawing specifies a measured grade; the factory's quotation specifies a finish. Two documents, two languages, and nobody in between to translate. Granite paving slip resistance is not a property of the stone itself. A grade is what one slab, one laying method and one test point produce on the ground; a finish is what a flame, a hammer head or a saw blade did in the shed. One can only be measured, the other can only be made. Write a grade code on your enquiry and the answer comes back as 'flamed'. Both sides believe the point is settled, and the argument starts the first time somebody puts an instrument on the paving. So the first step is not choosing a stone. It is separating the two languages and giving each its own box on the form.
Why does one slab give three different slip numbers?
Because the three methods measure three different actions, and the numbers do not convert into each other. The ramp test puts a person on a tilting test bench, walking up and down while the angle is raised step by step until the foot starts to slip, which returns a class. The pendulum test sweeps a rubber slider across the face on the end of an arm and returns a single friction reading. A portable device pushes a small slider over the surface and returns a continuous number, so several points can be taken across one area. The five standards fall into the three method rows of the table: DIN 51130 (oil under the shoe) and DIN 51097 (water under a bare foot) on the ramp, EN 14231 and BS 7976-2 on the pendulum, ANSI A326.3 on the portable device (examples only, and your project country's current edition governs). The same slab tested three times on the same day gives three numbers that are all correct, and none of them converts into another. Putting a portable reading where the drawing asks for a class is where this goes wrong.
| Method | Action it imitates | Medium | Form of the number | Can it be done on site? |
|---|---|---|---|---|
| Ramp test | A person walking up and down an inclined face | Oil under the shoe, or water under a bare foot | One class, read in steps | No: it needs a tilting test bench, so slabs go to a lab, and what comes back describes the sample, not this paving |
| Pendulum test | A rubber slider swept across the face | Dry face, or a face sprayed with water | One friction reading | Yes; the instrument travels, the test point is the hard part |
| Portable device | A small slider pushed over the face | Wet face, wetted before the reading | A continuous number, averaged over points | Yes; fast per point, suits spot checks over a large area |
Can the factory guarantee the grade? It can guarantee two other things
A grade is not an ex-works value: a site method measures this paving, a lab method measures the sample, and neither number is one the factory can sign for. The same finish on a sloping path and on flat ground does not measure the same on site; the same paving after rain and after washing is not the same number; the same spot on a new slab and on a slab two years old is not the same number. At the factory there is no test point and no ground, only slabs; a sample sent to a lab comes back with a class, but that class travels with the sample, not with the site. The first of the two is the process: how fast the flame travels, how dense the hammer pattern is, how deep the saw marks sit, whether the arris is eased. Those are workshop parameters, repeatable and writable into a work order. The second thing is the sealed sample: one slab from the same block and the same finish, dated and tagged with the batch number, one copy kept by each side, compared on arrival for relief depth, hammer density and how sharp the arris still is. We were founded in 2019, run our own quarry and 20 large saws, and hold Sesame Black (G654) samples by batch at 30mm.
Why can't a dry reading be used to accept outdoor paving?
Because a dry reading is the slab at its best, and outdoor disputes always arrive after rain and after washing. With a film of water on the face, water sits between the sole and the stone, and it pools wherever the fall is short; walking on that is a different event from walking on a dry face. So the word 'wet' is not enough on the form. Write the pre-test condition: how long after wetting the reading is taken, and whether the film is still standing or already drawing back; whether the area is washed or degreased first; whether sealer or curing compound is still on the face, because residue makes water bead and moves the reading; and whether the sample is new or two years old, since an old face has pores packed with dirt and arrises worn round, which is a different surface altogether.
Can polished slabs be used outside? Judge by position, not by stone type
Yes, but only where water cannot stand and nobody has to walk through it; once a slab holds water and people walk on it, the finish changes. Judge by position, not by stone type. The case for polished against flamed is not repeated here, the link at the end covers it, and the only question answered here is which positions must change finish. Across one plaza the broad flat paving is the base finish and does not need upgrading for slip; four positions do need their own decision, listed below. Those four positions are usually a small share of the area, but they are exactly where people change gait: on a slope, at a step nosing, over a threshold, around water. None of them is solved by picking a different stone; it is solved by writing the face treatment and the fall as two separate items on the order.
| Position | What happens there | How it is set |
|---|---|---|
| Broad flat paving | Adequate fall moves water off; it does not stand on the face | Base finish across the whole area, no upgrade for slip |
| Sloping path | Water runs as a sheet and separates sole from stone | Finish must change; fall direction and finish set together |
| Step nosing | The foot lands at an angle and the nosing takes water first | Finish must change; nosing eased, no sharp arris left |
| Entrance transition | Dry inside, wet outside, and the sole carries water both ways | Finish must change; same finish on both sides of the line |
| Water feature edge | Spray and permanent damp grow algae on the face | Finish must change, and drainage is left in that strip |
What goes on the enquiry and on the acceptance form?
Six lines, and none of them may be left blank. The blank line is where the argument will happen. These six lines are not all written for the factory: the finish and the method are settled before the order, and the other four are left for whoever accepts the work two years from now, when the person, the season and the instrument have all changed and only the words on the form have not. Read each line against one question: can somebody else check this again? Any line that needs an adjective to be understood is a line that is not finished. The six are not parallel either: the pre-test condition and the test points settle the acceptance, and who tests plus the sample settle who speaks after something goes wrong. The two most often left blank are the third and the fifth: without a pre-test condition, each side has its own defensible reading after rain; without naming who tests, a site reading moves with the person, and a lab report moves with the laboratory and the batch sampled.
| What to write | Why |
|---|---|
| The finish, fixed position by position: flamed for the broad flat paving, and a separate finish named for the sloping path, step nosing, entrance transition and water feature edge, with the arris noted as eased or not | The same word means two different roughnesses at two factories; writing it is what makes comparison with the sample possible |
| Test method and edition: the full set for the project country's current edition — DIN 51130 / DIN 51097 for the ramp, EN 14231 / BS 7976-2 for the pendulum, ANSI A326.3 for the portable device — with the edition year | The same slab under another method gives another number; the ramp method is lab-only, while pendulum and portable instruments can travel, so the two are written differently |
| Pre-test condition: how long after wetting, whether the film is still standing or already drawing back, whether the face is washed or degreased, whether sealer remains on it, and whether the slab is new or old | Nobody accepts a dry reading on paving after rain |
| Test points: for a site method, which positions and how many points each, and whether slopes and nosings are included; for a lab method, how many slabs are sent and from which batch | Testing only the flat paving means the riskiest positions were never tested |
| Who tests: for a site method, both sides on site with a joint record, or a third party attending; for a lab method, which laboratory, the date sent and the report number | A site reading moves with the person and the instrument; a lab report moves with the laboratory and with the batch sampled |
| The sample governs: date sealed, batch number, one copy each | Reproducing a finish runs off the sample, not off adjectives |